The invention is based on an exhaust turbocharger as described in the preamble to claim 1. This turbocharger has an exhaust gas turbine and a turbine casing within which a volute inlet flow duct is formed. The inlet flow duct carries an exhaust gas flow to the exhaust gas turbine. In this arrangement, the flow may impinge at right angles (radial turbine) or obliquely (mixed-flow turbine) onto the leading edges of the turbine blading. The inlet flow duct is connected to an exhaust gas conduit of an internal combustion engine by means of an inlet connection. Because the exhaust gas flow is subjected to quite substantial perturbations (depending on the configuration of and loading on the internal combustion engine), the exhaust turbocharger has means of transmitting the flow in such a way that it is substantially perturbation-free when it impinges on the exhaust gas turbine.
Such an exhaust turbocharger is, for example, described in EP 0 131 719 B1, in particular the embodiment shown in FIGS. 1 and 2. In this exhaust turbocharger, the exhaust gas flow is carried through a guide apparatus with rotor blades which can be pivoted in the radial direction. Compensation is provided for pressure and velocity fluctuations in the flow by pivoting the rotor blades and, by this means, a uniform incident flow onto the exhaust gas turbine is achieved.
Such a guide apparatus is relatively complex and, in addition, requires a comparatively large amount of space.
It is also known art to smooth swirled airflows by means of flow straighteners before the air inlet of the compressor of an exhaust turbocharger in order to increase the efficiency of the turbocharger. On this point, reference is made to a flow straightener described in DE 42 01 677 C1. This flow straightener consists of a sprung sheet-metal strip, which is configured with a plurality of bends in its longitudinal extent in order to form air conduction ducts and which is inserted so that its position is fixed with pre-loading in the air induction pipe of the compressor. A free end of the sheet-metal strip is used to form the boundary of two internally located air conduction ducts relative to one another. This flow straightener would not be suitable for smoothing an exhaust gas flow because the strongly swirled, hot exhaust gas can easily excite the free end of the sheet-metal strip to undesirable vibrations.
The invention, as defined in the claims, achieves the objective of providing an exhaust turbocharger of the type mentioned at the beginning and which, despite simple construction, is characterized by a high level of reliability even under more difficult operating conditions.
In the exhaust turbocharger according to the invention, the guide means are configured as a flow straightener and the flow straightener is arranged in the inlet connection of the turbine casing. This achieves the effect that the exhaust gas emerging from the internal combustion engine is carried via an inlet flow duct to the exhaust gas turbine in the form of a straightened flow with little swirl. This is particularly advantageous if the internal combustion engine is embodied as a gas engine or a diesel engine and exhausts to the turbocharger an exhaust gas flow which is strongly swirled and/or is subjected to large fluctuations in time. The vortices present in such a flow no longer reach the exhaust gas turbine or a guide apparatus provided upstream of the exhaust gas turbine and preferably embodied as a nozzle ring. No undesirable vibrations, which could lead to a failure of the turbocharger, can now be excited in the exhaust gas turbine and/or the guide apparatus.
If the flow straightener has guide elements having, in each case, a section which has a mainly plate-shaped configuration and which extends, in the region of the inlet connection, in the volute direction and at right angles to the latter in the radial direction, the tangential component of the exhaust gas flow in the exhaust gas conduit between the internal combustion engine and the turbocharger is decisively reduced. This prevents, to a high level of reliability, an exhaust gas flow subject to severe swirl being carried to the exhaust gas turbine.
It is recommended that the ends of the guide elements should have a free configuration because, in this way, thermal expansions caused by the hot exhaust gases cannot build up stresses in the flow straightener. The free ends of the guide elements are expediently arranged on a circle whose radius is, as a maximum, half the radius of the inlet connection of circular cross section. The tangential components of the exhaust gas flow are then greatly reduced but the pressure losses in the exhaust gas flow are, at the same time, kept small by this means.
If the casing of the exhaust turbocharger is produced by casting, it is then advantageous, from the point of view of manufacturing technology, to mold the guide elements into the inner wall of the inlet connection.
If the casing already exists, the flow straightener can then be installed particularly simply if it is embodied as an assembly unit which can be fixed in the inlet connection and has partial ducts of trapezoidal cross section extending in the direction of the tube axis of the inlet connection. First partial ducts should open at the small base side of the trapezium into a duct which is arranged centrally in the inlet connection and extends in the flow direction of the exhaust gas. In this way, the guide elements are configured with the free ends which are important with respect to the thermal stresses in the flow straightener and, at the same time, the exhaust gas flow is straightened particularly effectively on the radial arms of the trapezium. Improvement to the straightener function, while maintaining the free ends of the guide elements, is achieved by means of a second partial duct arranged between two of the first partial ducts, which second partial duct has a closed configuration on the small base side of the trapezium.
A fastening element configured as a tab is preferably formed on a wall part, of the flow straightener, which bounds the large base side of the trapezium. Such a fastening element can be carried in a simple manner by angling on a fastening flange of the inlet connection. In this way, the flow straightener can be fixed in the turbocharger in such a way that it is secured against rotation and displacement.
The flow straightener can be easily formed, from the manufacturing technology point of view, from a sheet-metal strip which is plastically deformed a plurality of times by bending in the longitudinal direction, the ends of which sheet-metal strip are connected to one another by the material becoming bonded together, by welding or the like. Because of the connection of the ends by material bonding, a mechanically stable body is formed which can satisfactorily withstand the forces transmitted by the exhaust gas flow. The metal strip is preferably formed from an alloy which is oxidation-resistant and corrosion-resistant at the exhaust temperature of the exhaust gases, on the basis of nickel-chromium or nickel-chromium-iron or the like.
If the flow straightener has an elastically deformable configuration in the radial direction and is manufactured oversize relative to the internal diameter of the inlet connection, it can be fixed under preload in the inlet connection by means of an appropriate assembly tool.